Development of the very low angle detector for epithermal neutron scattering at low momentum transfers

The design and prototype tests of the Very Low Angle Detector (VLAD) for epithermal neutron scattering at low momentum transfers are presented. The detector will be installed on the VESUVIO spectrometer at the ISIS spallation neutron source for so-called High-energy Inelastic Neutron Scattering (HINS) experiments. It will consist of an array of Resonant Detectors (RD) covering the scattering range 1/spl deg/-5/spl deg/. The results of prototype tests with a reduced set of RD units demonstrate the feasibility of the measurement in the adverse environmental conditions of ISIS. Simulations of particle interactions using the GEANT4 code show that the envisaged detector design can meet the requirements in terms of efficiency, cross-talk between detection channels and neutron scattering events in the detector.

I. Introduction

Epithermal neutron scattering experiments can access a unique range of exchanged wave vectors, , and energy transfers, , for the investigation of condensed matter. Routine measurements are performed nowadays with epithermal neutrons in the VESUVIO [1] spectrometer at the ISIS pulsed spallation neutron source. VESUVIO is an Fig. 1. Principle of an inverse geometry time of flight spectrometer based on the resonant detector approach. The incoming collimated neutrons are scattered by an angle in the sample before being captured in an analyzer foil. The prompt gamma rays following neutron capture are recorded in a detector attached to the analyzer foil. The scattering kinematics is uniquely specified by the final energy and time of flight of the scattered neutrons. For the VLAD detector on VESUVIO , and . inverse geometry time of flight spectrometer (see Fig. 1) where a “white” neutron beam is scattered off a sample and the scattering process is reconstructed by recording both the final energy and time of flight of the scattered neutrons. The range of exchanged wave vectors and energy transfers presently accessible is and corresponding to scattering angles above 20$#x0023;x00B0;. This is ideal for a broad range of investigations on condensed matter systems such as hydrogen bond and metal hydrides [2], metals [3], glasses [4], and quantum fluids and solids [5]. Yet there is an interest in extending the range of scattering parameters to the kinematical region of low () wave vector transfers coupled to high energy transfer (typically ). In this range – which we call High-energy Inelastic Neutron Scattering (HINS) – new and unique experimental investigations would become possible such as dispersion relations of magnetic materials [6], semiconductors [7], high lying molecular rotational-vibrational states, molecular electronic excitations and electronic levels in solids [8]. HINS experiments require the detection of epithermal neutrons at scattering angles as low as 1$#x0023;x00B0;–5$#x0023;x00B0;. To this aim a new detector bank, the Very Low Angle Detector (VLAD), has been designed and will be installed on VESUVIO.